Electrochemical conversion processes applied to metals such as aluminum conventionally involve the etching of metal surfaces as a preliminary treatment. For instance, prior to anodization of aluminum or coloring of stainless steel, etching is performed for various purposes such as the removal of unwanted materials from metal surfaces, activation thereof, and roughening of the same. Etching applied for these purposes is roughly divided into two types, chemical etching involving the immersion of the work in an etchant solution, and electrolytic etching in a bath. Electrolytic etching is conventionally performed with a d.c. current applied to the work serving as an anode, or with an a.c. current or alternating pulsive current being applied to the work. The latter method, referred to as "a.c. etching", is popular today chiefly because it is capable of producing a uniform surface on the work and because it allows for simple post-treatments.
The present invention basically relates to an electrolytic treatment that involves the use of an a.c. current or alternating pulsive current. This method of etching has been performed by various techniques. In electrolytic etching of aluminum, a bath with a pH of 1 to 8 such as aqueous sodium chloride or hydrochloric acid that contains chloride ions is commonly employed and an a.c. or alternating pulsive current is applied at a density of 10 to 100 A/dm.sup.2 to a graphite counter electrode. This technique is most common because it enables efficient etching operations. However, the graphite used as a counter electrode is less conductive than metals and in order to permit operations at current densities as high as 10 to 100 A/dm.sup.2, the electrode must be made very thick and large and this increases the size of the equipment.
A further problem with graphite is that it is not as convenient to handle as are metals and that it cannot be freely worked into desired shapes. Besides this problem, the graphite electrode is generally porous and either absorbs the liquid electrolyte or undergoes electrolytic reactions in the electrode during service. As a result, it gradually loses its surface shape and is unable to be used consistently for a prolonged period. Furthermore, the need to increase the distance between the electrode and the work results in an increased electrolytic voltage and hence in increased power consumption.
With a view to solving these problems, a method has been proposed that uses an electrode that is made of a valve metal as exemplified by titanium, a corrosion-resistant metal. This method effectively solves the problems with the graphite electrode, such as large size, large work-to-electrode distance and high power consumption. However, the valve metal, as its name implies, provides a valve action by which it forms a passivated film on its surface to retard current flow during anodic polarization and by which it admits free passage of current during cathodic polarization. Because of this "rectifying" action, the electrode cannot be employed in electrolysis with an a.c. current or alternating pulsive current without upsetting the balance between positive and negative polarities to cause adverse effects on the work. Stated more specifically, anodic polarization predominates over cathodic polarization with respect to the work and the waveform of the current applied is also distorted.
In order to solve these problems, an electrolytic treatment that employs a platinum-coated titanium electrode has been proposed. This method ensures a good balance between positive and negative polarities and appears to solve all problems by reducing not only the size of the electrode but also the power consumption. However, platinum is fairly vulnerable to a.c. current or alternating pulsive current and undergoes electrolytic reactions during use. Therefore, if the electrolyte contains chloride ions, chlorine and oxygen will evolve as a result of an anodic reaction and waste gas treatment will be required Furthermore, hydrogen evolving as a result of the cathodic reaction will embrittle the titanium substrate and the life of the electrode is inevitably shortened if the substrate breaks